The present invention relates to signal processing and more particularly to a method and an arrangement for producing a test signal with a predetermined signal-to-noise ratio and any selectable output frequency for measuring transmission systems that can be operated with different signal bandwidths.
With digital television signals, a variety of modulation formats are used today in the domain of cable, terrestrial and satellite transmission systems. Due to the multiplicity of symbol rates of the digital television signals, these digital television signals have different signal bandwidths. For example, there are transmission systems that can be operated with signal bandwidths of 4 MHz, 6 MHz, 8 MHz or 27 MHz.
For measuring such transmission systems test signals are required, which are given a defined noise level, or, in other words, have a defined signal-to-noise ratio C/N (C=carrier, N=noise) in decibels, and can be placed anywhere in a predetermined frequency range. Such test signals have to date been composed of separately generated modulated carrier signals and noise signals. However, the signal-to-noise ratio calculated from them is encumbered with the errors of the two signals and so measurements with such conventional test signals are imprecise.
There is a need for a simpler method, and one better suited to practice, for generating such test signals and a simple arrangement for carrying out such a method.
This and other needs are addressed by the present invention, in which a precisely calibrated signal-to-noise (C/N) test signal can be generated on a freely selectable output frequency to a desired signal-to-noise ratio of, for example, 20 dB, by attenuating the noise signal with a simple calibration line. This approach can be implemented with straightforward circuit technology. It is particularly advantageous to calibrate automatically carrier signal and noise signal levels to an equal ratio with a corresponding regulating arrangement, so the calibration is also constantly repeated during operation and any temperature influences or scattering of the carrier signal or of the noise signal are automatically corrected.
It has proved particularly advantageous to generate the carrier signal by modulation of the carrier frequency of a local oscillator and to remix both the carrier signal and the noise signal into the basic band with this same carrier frequency before comparing levels, so, irrespective of the adjusted carrier frequency of the carrier signal, the comparison of the levels and regulation of the levels take place at the constant output frequency of the inverse mixer.
Accordingly, one aspect of the present invention is directed to method for producing a test signal with a desired signal-to-noise ratio on a selectable output frequency for measuring a transmission system. In this method, a carrier signal, generated from a carrier signal, and a noise signal are remixed with the carrier signal in a bandwidth equal to or smaller than a smallest signal bandwidth of the transmission system. The levels of the carrier signal and the noise signal are adjusted to a predetermined ratio (preferably one) based on a measurement of the remixed carrier signal and the remixed noise signal. The noise signal is then attenuated and adding to the carrier signal until the desired signal-to-noise ratio is attained. In one embodiment, the carrier signal and the noise signal are remixed with the carrier signal to an intermediate frequency in the basic band.
Another aspect of the present invention relates to an apparatus for producing a test signal with a desired signal-to-noise ratio on a selectable output frequency for measuring a transmission system. The apparatus includes a first adjustable calibration line for generating a noise signal based on a noise source and a second adjustable calibration line for attenuating the noise signal and adjustable to a desired signal-to-noise ratio. A level detector is provided for receiving the noise signal and a carrier signal via a toggle switch, in which the first calibration line is controlled based on an output of the level detector so that a level of the noise signal is in a predetermined ratio with a level of the carrier signal.
In various embodiments, the apparatus may also include a modulator for generating the carrier signal based on a carrier frequency of a local oscillator, a mixer, supplied with the carrier frequency of the local oscillator, arranged between the toggle switch and the level detector, and/or a fixed frequency band filter coupled before the level detector, in which the band filter has a bandwidth equal to or smaller than a smallest signal bandwidth of the transmission system.
Still other aspects, features, and advantages of the present invention are readily apparent from the following detailed description, simply by illustrating a number of particular embodiments and implementations, including the best mode contemplated for carrying out the present invention. The present invention is also capable of other and different embodiments, and its several details can be modified in various obvious respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.